1. Field of the Invention
The present invention relates to a transistor. More particularly, the present invention relates to a thin film transistor for use in an organic light emitting display and a method for fabricating the same.
2. Discussion of Related Technology
Recently, various flat panel displays have been developed as substitutes for a Cathode Ray Tube (CRT) display which is relatively heavy and bulky. Examples of flat panel displays include a liquid crystal display (LCD), a field emission display (FED), a plasma display panel (PDP), and an organic light emitting display.
An organic light emitting display, among the flat panel displays, displays an image using an organic light emitting diode that generates light by the recombination of electrons and holes. Such an organic light emitting display has advantages of a high response speed and a low operational power consumption. The organic light emitting display can also be referred to as an organic electroluminescent display.
An active matrix organic light emitting display, among other organic light emitting displays, includes a substrate, a plurality of pixels formed on the substrate, and data and scan lines for providing video signals to the pixels. Each of the pixels is connected to a plurality of associated thin film transistors (TFTs). Each of the thin film transistors serves as either a switching element or a driving element.
Generally, a thin film transistor serving as a switching element (hereinafter, also referred to as “switching TFT”) is required to have different characteristics from those of a thin film transistor serving as a driving element (hereinafter, also referred to as “driving TFT”). A switching TFT generally requires a low threshold voltage and a low sub-threshold factor. A sub-threshold factor is also referred to as an “S-factor.” An S-factor is an inverse of a curve slope of a graph of source/drain current versus gate voltage. The graph generally indicates electric characteristics of a thin film transistor. A low S-factor indicates that there are only small defects in an interfacial surface between the gate insulation layer and the semiconductor layer of the thin film transistor. In other words, a low S-factor means that an interfacial surface is generally clean. On the other hand, a driving thin film transistor generally requires a relatively high S-factor because it should be capable of displaying gradation corresponding to video signals.
When a TFT has a low S-factor, a drain current may significantly change with a small variation of a gate voltage. Thus, it is difficult to perform a fine control of an electric current in a TFT. In addition, a TFT having a low S-factor may not be suitable for use as a driving TFT such as a TFT M1 shown in FIG. 1. In other words, a TFT having a small S-factor to work as a switching TFT may not work as a driving TFT.
In addition, a driving TFT used in an organic light emitting display needs gamma correction. Typically, an electric current ID flows through a drain electrode of a driving TFT to an organic light emitting diode in a pixel of an organic light emitting display. The electric current is proportional to the square of a gate voltage. Thus, the electric current and the gate voltage are in a nonlinear relationship. This nonlinear relationship is also referred to as the gamma characteristic. However, an organic light emitting display requires a linear relationship between these quantities and thus needs gamma correction to compensate the non-linear relationship.